This invention concerns improved herbicidal compositions containing a bipyridilium herbicide and at least one herbicide which has the ability to inhibit photosynthesis.
Bipyridilium herbicides are a well known class of herbicides having the general formulas ##STR1## in which P and Q, which may be the same or different, are C.sub.1 -C.sub.4 alkyl which may be substituted by hydroxyl, halogen, carboxyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, carbamoyl or N-substituted carbamoyl; X is an anion, preferably a halide, and n is an integer from 1 to 4. The two most prominent members are paraquat (1,1'-dimethyl-4,4'- bipyridilium ion and/or its salts) and diquat (1,1'-ethylene-2,2'- bipyridilium ion and/or its salts). Other bipyridilium herbicides are mentioned, for example, in Canadian patent 1,058,411. Bipyridilium herbicides, particularly paraquat, are non-selective contact herbicides which kill those plant tissues which carry out photosynthesis. As is well known, paraquat, for instance, is reduced in the presence of free electrons produced during the photosynthesis process to paraquat free radicals. In the presence of oxygen, these free radicals are quickly re-oxidized to paraquat, producing hydrogen peroxide and/or a so-called "superoxide", which cause rapid desiccation of plant tissues.
It is well known that paraquat is used most effectively in conditions of dim light, for instance on cloudy or overcast days, or towards evening. Under such conditions, photosynthesis proceeds more slowly, and the paraquat has an opportunity to translocate more through the plant, giving a more complete kill when it becomes activated. When paraquat is applied in bright sunny conditions when photosynthetic rate is high, paraquat reduction together with generation of toxic oxides is rapid, often so rapid that the paraquat effect remains localized near the point of its application. Application of paraquat under bright sunlight, therefore, can produce a quick effect, but significant re-growth can occur.
However, it is often impractical or inconvenient to apply paraquat under only those low light conditions which produce optimum activity. It has been found therefore, for instance by Headford, Pesticide Science, Vol. 1, pp. 41-42 (1970) that similar optimum effects can be obtained by combining paraquat with another compound, particularly another herbicide, which has the ability to inhibit photosynthesis. Headford found that a combination of paraquat and bromacil, at a 2:1 weight ratio, sprayed in full sunlight, gave effects similar to those from paraquat alone sprayed at 5:00 P.M. It also has been recommended to tank mix paraquat with a number of other herbicides, including at least one (linuron) which coincidentally is a photosynthesis inhibitor. In using tank mixes of paraquat with photosynthesis inhibiting herbicides, the best activity would be expected from the most water-soluble photosynthesis inhibitor, which would allow the paraquat to best translocate for a more complete kill. However, the most active photosynthesis-inhibiting herbicides are typically water insoluble.
In addition, simple tank mixes of paraquat with, for instance, wettable powder formulations of other herbicides, do not produce consistent effects in terms of optimum translocation and effect of paraquat.
As an improvement over tank mixes of paraquat and photosynthesis inhibiting herbicides, a formulation in the form of a stabilized suspension concentrate containing paraquat and diuron, in a 2:1 weight ratio, has been sold under the trademark Surefire. However, as with previously suggested combinations, the photosynthesis-inhibiting herbicide (in this case, diuron) is used in one-half the amount of the paraquat in order to provide the desired optimum paraquat translocation and herbicidal effect. It would be advantageous to obtain at least a similar effect with the use of much lower quantities of the photosynthesis inhibitor.